Tape servo pattern with enhanced synchronization properties

ABSTRACT

A magnetic tape servo pattern with different frequency signals to define the frame boundary. A significantly different frequency field is used to signal the transition from one frame to the next.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to the field of dynamic magnetic informationstorage or retrieval. More particularly, the invention relates to thefield of automatic control of a recorder mechanism. In still greaterparticularity, the invention relates to track centering using a servopattern. By way of further characterization, but not by way oflimitation thereto, the invention is a unique servo pattern withenhanced synchronization properties.

2. Description of the Related Art

Magnetic tape recording has been utilized for many years to record voiceand data information. For information storage and retrieval, magnetictape has proven especially reliable, cost efficient and easy to use. Inan effort to make magnetic tape even more useful and cost effective,there have been attempts to store more information per given width andlength of tape. This has generally been accomplished by including moredata tracks on a given width of tape. While allowing more data to bestored, this increase in the number of data tracks results in thosetracks being more densely packed onto the tape. As the data tracks aremore closely spaced, precise positioning of the tape with respect to thetape head becomes more critical as errors may be more easily introducedinto the reading or writing of data. The tape--tape head positioning maybe affected by variations in the tape or tape head, tape movement causedby air flow, temperature, humidity, tape shrinkage, and other factors,especially at the outside edges of the tape.

In order to increase data track accuracy, servo stripes have beenemployed to provide a reference point to maintain correct positioning ofthe tape with respect to the tape head. One or more servo stripes may beused depending upon the number of data tracks which are placed upon thetape. The sensed signal from the servo stripes is fed to a controlsystem which moves the head and keeps the servo signal at nominalmagnitude. The nominal signal occurs when the servo read gap is locatedin a certain position relative to the servo stripes. Referring to FIG.1, a one-half inch wide length of magnetic tape 11 may contain up to 288or more data tracks on multiple data bands 12. With such a large numberof data tracks it may be desirable to include up to five or more servostripes 13 to improve data read and write function performance. Servostripes 13 may utilize various patterns or frequency regions to allowprecise tape to tape head positioning thus allowing a data read head tomore accurately read data from data bands 12.

Referring to FIG. 2, a portion of a conventional servo stripe 13 isshown having two frames 14 and 15. A first frequency signal 16 iswritten across the width of servo stripe 13. As is known in the art, anerase frequency is written over first frequency signal 16 in apredetermined pattern such as five rectangles 17 in each of frames 14and 15. The five rectangles 17 in each frame result in nine horizontalinterfaces 18 between frequency signal 16 and erase patterns 17 as thetenth edge 19 along the bottom is ignored. A dashed line 21 illustratesthe alignment of a read gap 22 in a tape read head 23.

Referring to FIG. 2, if the alignment of read gap 22 with servo pattern13 is as shown, dotted line 21 passes along one of edges 18 and throughthe center of gap 22. If the servo pattern on the tape is passed rightto left over gap 22, then read gap 22 will alternate between readingfrequency 16 across the full width 24 of gap 22 in areas 25 andfrequency 16 across one half of read gap 22 and an erase frequency frompatterns 17 across the other half of width 24 in areas 26.

FIG. 3 shows the read frequency signals from one frame 14 or 15 as readby head gap 22 in FIG. 2. The amplitude of the signal is larger in areas25 where frequency area 16 passes over the full width 24 of head gap 22.The amplitude of the signal is about half as large in area 26 when onehalf of width 24 reads frequency area 16 and the other half reads erasepatterns 17. The servo control system in a tape drive uses the ratio ofthe full signal amplitude in field 25 to the half signal amplitude infield 26 to stay on track. For the on track position shown by dottedline 21 in FIG. 2 the ratio will be exactly one-half because one half ofread gap width 24 is over area 16 and one-half is over erase pattern 17.

Referring to FIG. 1 and FIG. 2, if tape 11 and hence servo stripe 13move down with respect to tape head 23, then, in field 26, more of area16 and less of pattern 17 will be over head gap 22. Referring to FIG. 3,if more of area 16 is read, then the signal in field 26 will increaseand this will be sensed by the controller. Conversely, if tape 11 andthus servo stripes 13 move up in FIGS. 1 and 2, then head gap 22 willsee less of area 16 and more of pattern 17 across width 24. Thus, thesignal in area 26 of FIG. 3 will decrease in amplitude proportionatelyto this movement. In this way the tape controller can sense the positionof the tape 11 with respect to the read gap 22 and move head 23 to keepthe head gap 22 aligned with the servo stripe along line 21. Thisalignment ensures precise reading of a data track in data bands 12 bythe data read head (not shown). While this system can result in moreprecise positioning of the tape head 23 with respect to tape 11, adifficulty can arise in that the controller must be able to determine inwhich field, 25 or 26, it is in at the time the signal is read. That is,there must be synchronization between the time the signal in field 25 issampled and the time the signal in field 26 is sampled.

Referring again to FIG. 3, the change in signal amplitude in moving toor from field 25 to or from field 26 could be used to determine in whichfield/area on the servo stripe read gap 22 is located. That is, if thesignal drops to about one-half amplitude, it can be assumed that gap 22is sensing movement from field 25 into field 26. Conversely, if thesignal amplitude approximately doubles, it can be assumed that gap 22 issensing tape movement into field 25 from field 26. However, this methodis prone to error for a number of reasons. If head gap 22 is alignedsuch that it passes between erase patterns 17, only signal 16 will beread and there will be no amplitude change in the signal from areas 25to areas 26 or vice versa. In effect the control system which positionsthe head with respect to the tape is lost. The tape controller systemdoes not know whether gap 22 is in region 25 or 26. Another possiblesource of error is noise. Because the signals are analog and contain asignificant amount of noise, it may be difficult to determine the changein amplitude as gap 22 senses movement from field 25 to field 26 andvice versa. It would be desirable to have a system in which the servocontrol circuitry could reliably determine in which region read gap 22is then located.

SUMMARY OF THE INVENTION

The invention is a novel servo track pattern in which a synchronizationfrequency area is utilized as a transition area from one frame toanother. A higher frequency in the synchronization signal area on theservo stripe is used as the transition area to define the frameboundaries. That is, the large frequency difference is used as thecriterion to improve frame edge detection.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an illustration of multiple servo stripes and data bands onmagnetic tape;

FIG. 2 is an illustration of a servo pattern including multiple erasebands;

FIG. 3 is a graph of the analog signal generated from the servo patternof FIG. 2;

FIG. 4 is an illustration of a servo pattern including a synchronizationfrequency area;

FIG. 5 is a graph of the analog signal generated from the servo patternof FIG. 4 converted to a digital signal along with the clock pulses andthe detection signal; and

FIG. 6 is a block diagram of signal conversion and detection circuitry.

DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring to the drawings wherein like reference numerals denote likestructure throughout each of the various drawings, FIG. 1 illustratesmultiple servo stripes 13 written onto a given tape portion 11 to allowprecise positioning of data bands 12 with respect to a data read head(not shown). FIG. 4 illustrates a servo pattern to be written as servostripes 13 onto tape 11. Referring to FIG. 4, a synchronizationfrequency signal is written on a first area 27 across the width of servostripe 13. A different frequency signal is written on a second area 28across the width of servo stripes 13. First area 27 and second area 28together comprise one frame 14. Synchronization frequency region 27 andservo modulation frequency area 28 are then alternately written ontoservo stripe 13 in successive frames 15, etc. along a length of tape 11.As in FIG. 2, a third frequency signal, which may be, for example, anerase frequency signal, is written in a predetermined band pattern ineach frame over second area 28. In the preferred embodiment, the erasefrequency pattern is written in the form of parallelograms 17 which maytake the form of a square or rectangle. During operation of the tapedrive the location of the tape head relative to the tape is controlledby servo readers which monitor the output signal when the reader ispositioned at the edge of erase bands 17 as was discussed above withrespect to FIG. 2.

Referring to FIG. 4, fields 25 and 26 in frames 14 and 15 may beidentical to those in FIG. 2. However, in accordance with the invention,the signal frequency in area 27 is approximately double that of secondfrequency area 28. Thus, referring to FIG. 5, the frequency in field 29of an analog signal 30 sensed by the read gap 22 is approximately doublethe sensed frequency in adjacent fields 25/26/25. This frequencydifference allows use of a criterion other than change in amplitude todetect the transition to or from a frame (e.g. frame 14 to frame 15 inFIG. 4). That is, the frequency change from field 29 to field 25/26/25and vice versa enables frame edge detection which is less subject tonoise and errors than a system such as shown in FIGS. 2 and 3 whichrelies on detecting the amplitude change in moving to or from fields 25and 26.

Referring to FIGS. 5 and 6, the analog signal 30 in FIG. 5 is convertedinto a digital signal 31 by a data qualifier circuit 32 as used in dataread channels. Data qualifier circuit 32 may be a zero crossing detectoras is known to one skilled in the art. The half periods defined by eachhigh and low pulse in digital signal 31 are proportional to the periodof analog signal 30. The frequency of digital signal 31 in field 29 ofFIG. 5 is different than the frequency of digital signal in fields 25,26, 25 while the frequency of the signals in fields 25 and 26 issubstantially identical. Because it is a digital signal, the amplitudedifferences between the pulses in fields 25 and 26 and 29 are notsignificant.

The detection of the frame edge in FIG. 5 is accomplished by the circuitshown in FIG. 6. A clock signal 33 from a crystal oscillator 34 issupplied to a counter 35 along with digital signal 31. Clock signal 33is a very accurate high frequency signal. Counter 35 counts the number36 of clock pulses 33 during each half period of digital signal 31.Because the period of the signal in field 29 is about half the period ofthe signals in fields 25 and 26, the counter will count half as manyclock pulses during each half period of signal 31 in field 29 as duringa half period of signal 31 in fields 25 or 26. For example, referring toFIG. 5, the number of clock signals 33 during each half period definedby a high or low pulse in signal 31 in field 29 is about 4 while thenumber of clock signals during each half period in fields 25, 26, 25 isabout 8. Counter comparator logic 35 will generate a detection signal 37in response to a change in count of clock pulses 33 in consecutive halfperiods of digital signal 31. That is, compare logic circuitry incounter 35 is used to compare the number 36 of clock pulses 33 which arecounted during each half period defined by a high or low pulse indigital signal 31. For example, upon detecting the decrease in thenumber 36 of clock pulses 33 during each half period of signal 31 inmoving from field 25 in frame 14 to field 29 of frame 15, counter 35will generate a low detection signal at 38. Upon detecting an increasein count number 36 of clock pulses 33 for two successive half periodsmoving from field 29 in frame 15 to field 25 of frame 15, counter 35will generate a high detection signal at 39. Thus, the use of asynchronization signal with a frequency measurably different from thefrequency of the signal in the remaining portion of the frame allowsmore accurate detection of the frame edge.

As shown in FIG. 5, the transition in the number 36 of clock pulses 33may not change from, for example, precisely 8 to 4 and back to 8 again.Uncertainty in these transition counts may be accounted for by delayingthe detection signal one or more half periods of signal 31. In thepreferred embodiment as shown in FIG. 5, detection signal 37 changesstate (high to low or vice versa) after two successive half periods of apredetermined change in clock pulse counts 36 are detected. Thepredetermined change in count may be, for example, from two successivehalf periods with 6 or more counts to two successive half periods with 5or less counts (at 38). Conversely, a change in count from twosuccessive half periods to a count of 5 or less to two half periods withcounts of 6 or more results in an output signal at 39. In the preferredembodiment, there is a delay in generating the detection signal 37 bycomparing counts for two successive half periods such that the totaldelay is one period. This known delay may be accounted for by thecontroller in utilizing detection signal 37. The count frequency and therequired number of counts to define a transition may be varied accordingto engineering design considerations as will be apparent to one skilledin the art. Likewise, while two half periods of frequency change areused in the preferred embodiment for redundancy/accuracy purposes, anynumber of half periods of change may be used in accordance with theinvention.

While the invention has been described with respect to a particularembodiment thereof, it is not to be so limited as changes andmodifications may be made which are within the full intended scope ofthe invention as defined by the appended claims. For example, whilespecific numbers of servo tracks and data tracks have been disclosed,the invention may be utilized with more or less servo or data tracks.While relative amplitudes and frequencies have been disclosed differentratios of these amplitudes and frequencies may be advantageously usedwithout departing from the scope of the invention. Similarly, whileparticular tape servo patterns have been disclosed, more or less fieldsand different types of patterns may be employed without departing fromthe scope of the invention.

What is claimed is:
 1. A method of reading a servo track on a tapehaving a given width and a given length, the tape having a series oflongitudinal adjacent frames extending longitudinally along a part ofthe given length of the tape and laterally across a part of the givenwidth of the tape, each of the frames having a given width and includingfirst and second longitudinal adjacent portions each extending laterallyacross the given width of the frame, the first longitudinal adjacentportion having a first frequency signal and the second longitudinaladjacent portion having a second frequency signal, the secondlongitudinal adjacent portion further including a third frequency signalwritten onto a rectangular area over the second frequency signal,wherein the rectangular area defines a servo track extendinglongitudinally across the second longitudinal adjacent portion of aframe thereby separating top and bottom portions of the secondlongitudinal adjacent portion, wherein the first and second portions ofadjacent frames are adjacent to one another, wherein the servo trackextends longitudinally across the second longitudinal adjacent portionof the frames along the part of the given length of the tape, the methodcomprising:moving the tape longitudinally across a tape head; readingthe servo track with a servo read element of the tape head as the tapemoves longitudinally across the tape head, wherein the servo readelement only reads the servo track; and detecting a transition along theservo track between adjacent frames from the change in frequency betweenthe first and second frequency signals.